Mature writer supporting conclusion of younger one (5)
I believe the answer is:
ripen
'mature' is the definition.
(to ripen is to mature)
'writer supporting conclusion of younger one' is the wordplay.
'writer' becomes 'I' (the person who is writing these clues).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'conclusion of' says to take the final letters.
'one' becomes 'pen' (I can't justify this - if you can you should believe this answer much more).
The final letter of 'younger' is 'r'.
'i' after 'r' is 'ri'.
'ri'+'pen'='RIPEN'
Can you help me to learn more?
(Other definitions for ripen that I've seen before include "Grow older" , "Become ready to pick" , "Become ready to eat" , "Make ready" , "Become mature" .)